Hot-filament chemical vapor deposition (HFCVD) constitutes one of the most viable techniques in low pressure diamond film fabrication. To improve the quality and yield of CVD diamond films, it is desirable to measure a variety of key deposition parameters during film growth, including filament temperature and film growth rate. Filament temperature is known to affect the chemical composition of the deposited gas which, in turn, affects the film quality and yield. The growth rate of the deposited film also corresponds to the resultant film thickness which must be varied according to the desired application.
Various methods have been proposed for the measurement of filament temperature alone through the use of a pyrometer which determines the subject temperature based upon radiation emissivity. Prior to the Applicants' invention, however, no suitable technique has been disclosed for the concurrent in-situ monitoring and measurement of filament temperature and film thickness. Indeed, those skilled in the art will recognize that the harsh environment in which diamond films are typically grown has heretofore inhibited such processes.
U.S. Pat. No. 4,711,790 to Morishige, for example, discloses a device for monitoring the thickness and peak temperature of thick films grown onto a substrate during chemical vapor deposition. In operation, a pulsed optical beam emitted from an optical source is directed onto a predetermined area of a substrate where a thin film is to be deposited. A temperature monitoring unit is disclosed for receiving an infrared beam emitted from a heated CVD film during deposition. There is further disclosed a control unit for controlling the temperature monitoring unit to vary the temperature of the CVD film to a predetermined peak temperature. As disclosed by Morishige, the CVD film begins to partially deteriorate when the CVD film passes this peak temperature. Because the heat capacity of the CVD film is proportional to its thickness, a deterioration of the CVD film causes the film to fall past peak temperature. A temperature fall past peak temperature provides an indication that the CVD film has grown to the desired thickness. The control unit is therefore used to vary the intensity of the pulsed optical beam to maintain the peak temperature of the CVD film to within an optimum temperature range.
U.S. Pat. No. 4,024,291 issued to Wilmanns discloses a method for optically monitoring and regulating the thickness of a vapor deposited layer formed onto a silicone substrate. Wilmanns discloses the commonly known spectroscopic technique which entails using a photoreceiver to collect, and then transform into electrical output signals, light beams reflected off of the surface of a CVD layer. The electrical signals are passed through two differentiating units and then further processed to determine the build up of the CVD layers on the substrate.
U.S. Pat. No. 4,331,725 issued to Holland discloses a method for controlling the deposition of thin films onto a substrate by measuring the optical reflectance or transmittance from a deposited film. As disclosed by Holland, during the measurement process, a pair of monitors comprising light detectors collect and measure light reflected from a film being deposited onto a substrate. The reflected light is transformed into an electrical signal having magnitude corresponding to the reflectance or transmittance of the film being deposited onto the substrate. Holland further discloses a microprocessor for calculating the quotient between the electrical signal and a signal corresponding to the reflectants or transmittants from a film being deposited onto a separate quartz crystal. Significantly, the crystal is assumed to be subject to vapor deposition of the same material and at the same rate. Those skilled in the art will recognize, however, that such crystals are not suited to the high temperatures typical of CVD diamond deposition.
U.S. Pat. No. 4,525,376 issued to Edgerton, discloses a method for continuously controlling the thickness of a layer of material deposited onto a substrate through the use of an optical detector disposed inside of a glow discharge chamber. The complex system disclosed by Edgerton requires an optical detector, i.e. an optical pyrometer, for measuring light reflected from thin films of material being deposited onto a substrate. In operation, the thickness of the layer being deposited onto a substrate is determined by comparing the intensity of the reflected beam as a function of wave length with that of an incident beam being directed towards the substrate from an external light source.
U.S. Pat. No. 4,959,244 issued to Penney, et al discloses a method for measuring temperature of CVD films deposited on a workpiece. A complicated temperature measuring system is disclosed which collects radiation emitted from the workpiece surface which, in turn, is passed through a spectrometer. The spectrometer is used to detect shorter wave length light toward the blue edge of the spectrum for determining a surface temperature signal. The detector system disclosed by Penney consists of a prism, focusing optics, an intensified linear array detector, and a photon counting device.
Significantly, none of the prior art references teach or suggest Applicants' method or apparatus for concurrent in-situ monitoring and control of filament temperature and film thickness. Such measurement and control is of critical importance to the operator during hot filament chemical vapor deposition of diamond films. In each of the references disclosed above, the monitoring of film thickness also requires an additional light source such as a laser or white light, which is obviated by Applicants' design.